Consider the unit tangent vector \(\nu \)  to the geodesic line \(\Gamma (\mathbf {x},\mathbf {x}_{\alpha })\)  at the point \(\mathbf {x}\)  [1]}
\(\nu =\frac{\nabla _{\mathbf {x}}\tau \left( \mathbf {x},\mathbf {x}_{\alpha }\right) }{\sqrt{\varepsilon _{r}\left( \mathbf {x}\right) }}.\) 
